The "AWS" that Barr refers to is "Amazon Web Services," the mega-popular cloud computing service that's also become Amazon's most profitable division. Like lots of other big companies, Amazon relies on AWS to power its whole business, including the Amazon.com website, the mobile app, and the Amazon Echo home assistant.

Barr says that circa 2002 or so, Amazon would just buy extra hardware for big events like the holiday shopping season, only to let it go disused after the fact. That's a big part of why AWS is such a quantum leap: With cloud computing, you can just add more computing capacity on-demand to deal with traffic spikes and the like.

Just another customer

To that end, AWS helped Amazon gracefully deal with Prime Day's massive flood of deal-seeking shoppers — a million people downloaded the Amazon app for the first time on Prime Day, Barr writes, in addition to the increased traffic to the Amazon.com website. And they didn't have to actually buy a single extra server.

Intriguingly, Barr says that AWS treats Amazon.com like "just like any of our other big customers," which is to say that they don't get any special treatment when it comes to things like customer support. Instead, Barr says, AWS and Amazon.com act as "business partners," and communicate like they would with any outside entity.

"Sticking to this somewhat formal discipline helps the AWS team to improve the support plans and the communication processes for all AWS customers," Barr writes.

AWS is certainly not the only game in town, with Microsoft Azure and Google Cloud Platform both gunning for Amazon's top spot in the cloud computing market. But the fact that Amazon itself uses AWS to power even mega-events like Prime Day is a good illustrative example of why so many customers rely on it in the first place.